This invention relates generally to magnetoresistive (MR) sensors. More particularly, it relates to a method of making MR sensors having a trackwidth narrower than 0.2 micron.
Magnetoresistive (MR) sensors for detecting and measuring magnetic fields find many scientific and industrial applications. Prior MR sensors include anisotropic magnetoresistive (AMR) sensors and giant magnetoresistive (GMR) sensors, in which a sense current flows along, or parallel to, planes of the ferromagnetic elements. Prior MR sensors also include magnetoresistive tunnel junction (MTJ) sensors, in which a sense current flows perpendicular to the planes of the ferromagnetic elements through a dielectric barrier. The resistance of a MR sensor depends on the magnetization direction of the sensor. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the sensor, which in turn causes a change in resistance in the sensor and a corresponding change in the sense current or voltage.
The increasing areal density of magnetic storage media requires that the magnetic recording read/write heads be able to operate at ever-decreasing track widths (TW). Both the write element and the magnetic readback sensor of the recording head must be made smaller in order to achieve narrower data tracks. For example, in the highest areal density (xcx9c20 Gbit/in2) commercial products, the sensor TW, which is defined by optical lithography and ion beam milling, is approaching 0.3 micron. It is envisaged that in order to make heads suitable for recording densities of 100 Gbits/in2, the sensor TW will need to be around 0.13 micron.
At present, magnetoresistive (MR) heads are typically made by photolithographically defining the MR sensor from a continuous multilayer thin film. The MR sensor is often defined in two steps, one photolithographic step to define the TW dimension, and one lapping step to define the so-called xe2x80x9cstripe heightxe2x80x9d (SH) dimension.
In the photolithographic patterning of the TW, an undercut resist scheme is necessary for the formation of high quality junctions. The best MR sensors are fabricated using an optical lithography, bilayer resist pedestal technique. FIGS. 1a-1e illustrate the fabrication of contiguous junction hard bias MR sensors using this prior art bilayer resist pedestal technique. As shown in FIG. 1a, a bilayer resist pedestal structure includes an image resist layer 106 on top of an undercut polymer layer 104. For fabricating a GMR sensor, the bilayer resist structure stands on a GMR layer structure 102. The bilayer resist structure masks the active sensor region of the GMR layer structure 102 during an ion milling step which defines the sensor trackwidth edges as shown in FIG. 1b. The bilayer resist structure then serves as a liftoff mask for depositing the hard bias layers 108 and leads 110, which contact the edges of the sensor 102 as shown in FIGS. 1c-1d. As shown in FIG. 1d, a quantity of hard bias material 108xe2x80x2 and lead material 110xe2x80x2 is also deposited on the sidewalls and top of resist layer 106. However, this quantity of material is removed along with the resist layer 106 in a liftoff process described in a later step.
The undercut nature of the bilayer resist pedestal structure facilitates liftoff of the hard bias layers 108 and leads 110. The undercut also allows superior junctions to be formed between the hard bias layers 108 and the sensor 102 (by minimizing shadow effects from hard bias material 108xe2x80x2 deposited onto the resist 106 sidewalls and by eliminating the redeposition of milled material from the GMR structure 102 onto the resist 106 sidewalls). FIG. 1e shows the sensor 102 with contiguous hard bias layers 108 and leads 110 after a liftoff process for removing the bilayer resist pedestal structure.
Undercut bilayer resist systems of the type depicted in FIGS. 1a-1e can be fabricated using e-beam lithography rather than photolithography. The present sensor trackwidths of 0.3 micron are already beginning to push the resolution limits of I-line photolithography. Fundamental constraints such as the diffraction limit of light make photolithographically patterning sub-0.2 micron TW sensors with I-line radiation practically impossible. Electron beam lithography has no such resolution limits, which make it an attractive (but by no means the only) choice for patterning ultra-narrow trackwidth MR sensors. FIGS. 2a-2b are schematic diagrams illustrating the top and side views of a bilayer resist pedestal using an e-beam resist chemistry technique. An e-beam sensitive image resist layer 206 is deposited on a resist layer 204, which cannot be seen in FIG. 2a. The open regions 202 on the image resist layer 206 are formed by exposing those regions to an electron beam and then dissolving the exposed resist in a suitable developer. The undercut is then formed by using an appropriate developer to dissolve the bottom resist layer, where the undercut distance is determined by the develop time.
Despite the high resolution of e-beam lithography, the bilayer resist pedestal technique described above becomes intractable for achieving trackwidths narrower than 0.2 micron. One reason for this is that forming such narrow pedestals requires controlling the resist undercut to a precision of hundredths of a micron. More fundamentally, the bilayer resist pedestal cannot be extended below 0.2 micron because the top resist layer would collapse unless the amount of undercut used in the present bilayer resist pedestal structure were significantly reduced. This is not an option because reducing the undercut would adversely affect the liftoff process and the junction quality. One might imagine that those difficulties could be circumvented by reducing the thickness of the GMR layer, which would allow the thickness and width of the bilayer resist pedestal to be scaled accordingly. This is not an option, though, because significant reduction of the GMR layer thickness is not possible.
U.S. Pat. No. 5,079,035 issued to Krounbi et al. on Jan. 7, 1992, discloses a method for fabricating a magnetoresistive transducer with contiguous junctions between a MR layer and hard bias layers using a bilayer resist pedestal structure as described above. As stated above, the method disclosed by Krounbi et al. cannot fabricate a MR sensor with a trackwidth narrower than 0.2 micron.
A bridge structure is described in an article entitled xe2x80x9cOffset masks for lift-off photoprocessingxe2x80x9d by G. J. Dolan published on Jun. 21, 1977 in Applied Physics Letters. Using photolithography, Dolan fabricated micron-scale, suspended resist structures with micron dimensions in bridge width, bridge height, and in bridge separation from the substrate surface. By using this bridge as a mask for oblique angle thin-film deposition, small-area Josephson Junctions could be fabricated. However, the width of the bridge formed by this technique is 1.5 micron, which is far too large to be used for making MR sensors with narrow trackwidths.
There is a need, therefore, for a resist structure suitable for lithographically patterning MR sensors with trackwidths narrower than 0.2 micron.
According to an exemplary embodiment of the present invention, a fully undercut resist bridge structure to pattern MR sensors is formed by totally removing the bottom resist layer of a bilayer resist structure in the trackwidth region.
The fully undercut resist bridge structure is formed by using two polymer layers, with only the top polymer layer being sensitive to electron beam exposure and to the e-beam developer. Alternatively, short wavelength radiation (DUV, X-ray, and the like) could also be used to pattern the top polymer layer. In a preferred embodiment, the top polymer layer is made of an e-beam sensitive resist such as polymethyl methacrylate (PMMA). However, this imaging layer could be virtually any deep ultraviolet (DUV) resist (either positive or negative). The bottom polymer layer typically contains polymethyl glutarimide (PMGI).
E-beam exposed PMMA dissolves in a solution of isopropyl alcohol (IPA) and water. PMGI is not affected by this solution, regardless of whether it has been exposed to an electron beam. PMGI dissolves in a basic developer having concentrations of NaOH or KOH that do not affect the PMMA. Therefore, e-beam exposure and development of the PMMA layer will not affect the PMGI layer, and dissolving the PMGI layer will not affect the edges of the PMMA walls. When the top polymer layer contains DUV resist, a single developer, such as a basic developer of NaOH or KOH, can be used to develop both the e-beam exposed DUV and PMGI.
The fully undercut resist bridge of the present invention is fabricated by spinning PMGI to form a bottom thin resist layer on a substrate. A top thin resist layer is formed by spinning PMMA on the bottom resist layer. The top resist layer is then exposed to an electron beam in a bridge pattern defining the trackwidth of MR sensors. The E-beam exposed PMMA layer is then developed in a second developer, such as an IPA and water solution. The IPA/water solution removes the exposed PMMA but not the underlying PMGI material. Hence, by simply dissolving the PMGI layer for a sufficiently long time so that all the PMGI is removed in the trackwidth region, a dimensionally stable undercut bridge structure suspended above the substrate is formed, with a bridge width less than 0.2 micron, a bridge thickness less than 0.5 micron, and a bridge-substrate separation less than 0.1 micron.
The undercut resist bridge structure is used for lithographically patterning MR sensors. After patterning the bridge on top of the MR layer structures, the MR layer structure is subjected to ion milling in order to define the MR sensor TW. For a GMR sensor, magnetic hard bias layers are deposited in the passive regions at the abutting junctions on both sides of the GMR sensor to produce longitudinal bias for the sensor. Leads are then deposited on the hard bias layers for transmitting electrical signals. The resist bridge is then removed from the GMR sensor in a liftoff process.
Because it defines the gap between the bridge and the substrate, the thickness of the bottom resist layer is critical in the process flows outlined above. If the gap is too large ( greater than 0.1 micron), hard bias and lead material can be deposited under the bridge and on top of the MR sensor, creating a shunt that limits the device sensitivity. If the gap is too thin ( less than 0.04 micron) the redeposition of material onto the sides of the PMMA during ion milling may not be prevented. In addition, capillary action could cause the bridge to collapse during processing. The thickness of the top resist layer is also critical. This resist layer must be thick enough to give structural stability to the bridge. The thickness must also be sufficient to withstand milling through the GMR material layers of a thickness of between 0.04 micron and 0.06 micron. As the second resist layer becomes thicker, though, the resolution of the electron beam lithography will suffer. In general, for a 100 keV electron exposure, the minimum TW attainable is approximately one-tenth the resist thickness.
The suspended resist bridge structure is also suitable for defining narrow TW MTJ sensors. Following patterning of the bridge structure, ion milling is used to define the sensor. Next, insulating layers are deposited at the abutting junctions, before the hard bias layers are deposited. Additional insulating layers are then deposited on the hard bias layers. Using a liftoff process, the resist bridge is removed from the MTJ sensor. Finally, leads are deposited on the insulating layers in a separate process.
The method of the present invention produces MR sensors with trackwidth narrower than 0.2 micron. Furthermore, the method of the present invention allows MR sensors to be fabricated such that the ratio of the trackwidth to the sensor thickness is less than 4 to 1 (i.e., trackwidth is equal to 0.18 micron and sensor thickness is equal to 0.04 to 0.06 micron).
MR sensors produced by the method of present invention are incorporated in MR read heads. A MR read head includes a MR sensor, which is sandwiched between two gap layers and two shield layers.
MR read heads including MR sensors fabricated by the method of the present invention are then incorporated in disk drive systems. A disk drive system includes a magnetic recording disk connected to a motor and a MR read head including a MR sensor, which is fabricated by the method of the present invention, connected to an actuator. The motor spins the magnetic recording disk with respect to the MR read head, and the actuator positions the MR head relative to the magnetic recording disk.